As a result of the knowledge gained by the mass production of microwave ovens, the per kilowatt cost of microwave energy sources has fallen rapidly, opening a broad range of new applications in both commercial and industrial settings. One application is the use of microwave energy to initiate and sustain plasmas for use in plasma treatment processes, semiconductor etching, thin film deposition processes, and other processes.
While the conventional microwave oven is designed to be adequate to uniformly heat food products through the use of mechanical means to average out microwave energy non-uniformities and while taking advantage of the relatively long thermal relaxation times of the food products being heated, the same techniques cannot be used for uniformly exciting gases to create a plasma because of their short relaxation times. The fans and other mechanical "microwave dispersers" used in oven technology, are unable to assure a uniform dispersion of microwave energy, on a time scale appropriate to plasma excitation. To accomplish the uniform microwave excitation of a plasma other means have been employed.
U.S. Pat. Nos. 4,517,223 and 4,504,518 to Ovshinsky, et al, the disclosures of which are incorporated herein by reference, describe processes for the deposition of thin films onto small area substrates in a low pressure, microwave glow discharge plasma. Operation in the disclosed low pressure regimes not only eliminates powder and polymeric formations in the plasma, but also provides the most economic mode of plasma deposition. These patents describe low pressure and high energy density deposition utilizing microwave energy (i.e., near the minimum of the Paschen curve). However, the problem of uniformity of deposition over large areas remains unaddressed.
U.S. Pat. No. 4,729,341 to Fournier, et al, the disclosure of which is incorporated by reference, describes a low pressure microwave plasma process for depositing a photoconductive semiconductor thin film on a large area cylindrical substrate using a pair of radiative waveguide applicators in a high power process. The principles are limited to cylindrically shaped substrates, such as electrophotographic photoreceptors, and are not directly transferable to large area, generally planar substrates.
In an attempt to provide greater uniformity in the microwave energy radiated from the microwave applicators (and hence a more uniform deposition over a large planar surface), various "slow wave" microwave structures have been proposed.
U.S. Pat. No. 3,814,983 to Weissfloch and U.S. Pat. No. 4,521,717 to Kieser, the disclosures of which are incorporated herein by reference, both address the problem of non-uniformity by proposing various spatial relationships between the microwave applicator and the substrate being processed. Weissfloch discloses a slow wave waveguide structure that is inclined at an angle with respect to the substrate. Kieser discloses the use of two waveguide structures in an anti-parallel arrangement that superimposes the energy inputs of the two waveguides. This structure, however, results in an inefficient coupling of microwave energy into the plasma. Generally, the slow wave structures result in a rapid fall off of microwave energy as a function of distance transverse from the microwave applicator, leading to an inefficient coupling of microwave energy into the plasma.
The issue of microwave and plasma uniformity was also treated by J. Asmussen in "LOW TEMPERATURE OXIDATION OF SILICON USING A MICROWAVE PLASMA DISC SOURCE", J. Vac. Sci. Tech. B-4 (January-February 1986) pp. 295-298 and by M. Dahimene and J. Asmussen in "THE PERFORMANCE OF MICROWAVE ION SOURCE IMMERSED IN A MULTICUSP STATIC MAGNETIC FIELD" J. Vac. Sci. Tech. B-4 (January-February 1986) pp. 126-130. Asmussen describes a microwave reactor referred to as a microwave plasma disc source ("MPDS"). The plasma is in the shape of a disc or tablet having dimensions proportional to microwave wavelength. The plasma geometry is scalable to large diameters, potentially yielding a uniform plasma density over a large surface area. Asmussen describes a microwave plasma disc source which is designed for operation at 2.45 gigahertz, where the plasma confined diameter is 10 centimeters and the plasma volume is 118 cubic centimeters.
U.S. Pat. No. 4,481,229 to Suzuki, et al describes the use of electron cyclotron resonance to obtain a plasma having a relatively high degree of uniformity over a limited surface area. However, Hitachi fails to teach or suggest a method by which large area plasmas may be achieved.
U.S. Pat. Nos. 4,517,223 and 4,729,341, referred to above, describe the necessity of using very low pressures in very high microwave power density plasmas. Low pressure is needed to obtain high deposition rates and/or high gas utilization, and to economically carry out the plasma processes. However, the relationship between high deposition rates, high gas utilization, high power density, and low pressure further limits the utility of slow wave structures and electron-cyclotron resonance methods.
U.S. Pat. No. 4,893,584 to Doehler et al, the disclosure of which is incorporated herein by reference, discloses a microwave energy apparatus adapted to sustain a substantially uniform plasma over a relatively large area. The '584 Patent discloses a "leaky" microwave applicator structure whereby microwave energy is allowed to leak or radiate from the applicator through a plurality of apertures. The microwave energy apparatus disclosed by Doehler obviates the shortcomings inherent in slow wave microwave structures.
In generating a uniform, linear microwave plasma for the purpose of thin film deposition, the electric field strength of the microwave radiation that is coupled to the plasma is important in determining the size and the density of the plasma as well as in determining the thin film deposition rate onto a substrate.
One way of increasing the electric field strength within the plasma region is to increase the power capabilities of the microwave energy source. When the microwave source being used is a magnetron, the power of the microwave energy produced is proportional to the size of the device. However, increasing the size of the magnetron also increases the wavelength of the microwave energy generated, resulting in the need to increase the dimensions of the microwave applicator as well as many other parts of the apparatus.
In general, such dimensional changes are very costly. As well, they are impractical and may detrimentally effect the proper operation of the equipment. Hence, a microwave energy apparatus is needed that incorporates a more robust microwave applicator that can be easily adapted to changes in the wavelength of the microwave energy used.